122 



Water 



for the survival of a plant in relation to its water need may occur 

 during adult life. Plants differ greatly in the amounts of water that 

 they must absorb from the soil and transpire. Those with a high 

 water requirement, or transpiration ratio, are limited to habitats where 

 the supply of moistin-e is adequate. 



120 



Fig. 4.12. Top view of surface roots of cactus plant sliown on a 30-cni grid. 

 Almost the entire absorbing system of roots occurred in the upper 2 to 10 cm of 

 soil. (By permission from Phint EcoIo<^i/ by Weaver and Clements, 1938, Mc- 

 Graw-Hill Book Co.). 



